My oddball solution is "none and all of them".
I buy thin 23, 32 and 40 mm magnetic inserts from Litko. By themselves, they look like coin/washer bases and blend quite nicely into terrain. However, they also fit into the holes of plastic 30, 40 and 50 mm display bases. A medium-sized neodymium magnet under the plastic base will keep them together well enough for gaming, if the game system (and the opponent) absolutely requires it. Finally, the most common 23mm insert also fits on top of a flat 25mm round base, although that's rarely needed.
I still don't know whether this is a good idea. The as-is version is simple and looks good to me, though. Others are just a little bonus.
The price for small inserts is $12 / 100. I've seen worse.